For an amplifier, especially an operational amplifier, the response speed and the operation frequency depend on the slew rate (SR). The slew rate refers to the maximum rate of rise/fall of the output, and it is represented by the following formula 1.SR=k*ICS/C  (I)
Here, ICS represents the current value of the bias current in the operational amplifier. K represents a coefficient; k*ICS represents the current value of the operational current for charge/discharge of the capacitor for phase compensation, and it is also the current value of the operational current fed to the transistor pertaining to the amplification. C represents approximately the capacitance of the phase-compensating capacitor, and, strictly, it represents a capacitance that also includes the parasitic capacitance of the transistor.
Consequently, in order to increase the response speed and the operation frequency, that is, in order to increase the slew rate, one may simply reduce the capacitance of the phase-compensating capacitor or increase the bias current. However, a scheme for reduction of the capacitance of the phase-compensating capacitor can hardly be adopted in consideration of the stability (to prevent oscillation), and it is especially undesirable for a voltage follower. On the other hand, as the bias current is increased, the power consumption rises. For present amplifier applications, there is a high demand for lower power consumption, and, in many cases, is not effective simply increasing the bias current.
As an example, for a source driver that drives the signal line of a liquid crystal display, there is a D/A converter (hereinafter to be referred to as “DAC”) for converting the digital tone data that represent the display tone of each pixel to an analog tone voltage, and a voltage follower comprising an operational amplifier is set as a buffer amplifier in the final stage of the DAC for driving the signal line load with a relatively heavy capacitive property. In such application, the interval from the time of change of the input to the time it settles down to the target tone voltage (signal amplitude), that is, the settling time, is an important index in determining the usable frequency. Here, the settling time is determined by slew rate SR and the tone voltage (signal amplitude). For example, when slew rate SR is constant at 10 V/μs, the settling time when the signal amplitude is 4 V is 4 V/10 V/μs=0.4 μs, and the settling time when the signal amplitude is 1 V is 1 V/10 V/μs=0.1 μs. In this way, when slew rate SR is constant, the settling time depends on the signal amplitude, and, the higher the signal amplitude, the longer the settling time, and the poorer the settling characteristics.
Here, consider a scheme for changing bias current ICS proportional to slew rate SR. That is, when the signal amplitude is higher, bias current ICS is larger, and the slew rate SR and thus the rising speed become higher. When the signal amplitude is lower, bias current ICS is smaller. As a result, even when the amplitude is large, it is still possible to have a short settling time (improved settling characteristics) and, at the same time, it is possible to meet the demand for a lower power consumption.
In one aspect, the present invention addresses the aforementioned problems by providing an amplifier that can automatically and dynamically change the slew rate corresponding to the rate and amplitude of change of the input signal.
In another aspect, the present invention provides an amplifier wherein the settling time can be minimized, even for an input signal with a large amplitude.
In another aspect, the present invention provides an amplifier having low power consumption that can significantly improve the settling time characteristics.